Recently, in the field of servers and high-end computers, transmission capacity of input/output (I/O) function for communication between a CPU and an external interface has increased due to an improvement of performance. Meanwhile, in conventional high-speed electrical transmission, optical interconnect technology has been considered that enables high-capacity transmission by optoelectronic devices and optical signals.
Optical interconnect technology requires a smaller and lower-cost optical module than that for conventional backbone optical communication. Such an optical module is known that includes a circuit board that has a transparent material, an optical element (e.g., a light-emitting element and a light-receiving element) mounted on one surface, and an optical waveguide arranged on the opposite surface.
To further reduce fabrication cost of the optical module, a conventional transparent FPC board, on which an optical transmission element is mounted, is made of a thin film of, for example, polyimide, whereby low-cost and high-speed optical communication is enabled by providing such an optical module on, for example, a printed board in a server via an electrical connector.
In the field of the optical interconnect technology, a growing demand for fast optical transmission exceeding 20 Gbps requires utilization of an optical module on which a fast light-emitting/receiving element is mounted. However, coupling loss of signal light between the light-emitting/receiving element and the optical waveguide is large in the optical module described above.
For example, the larger the difference between the aperture of the light-emitting/receiving portion of the optical element and that of the inlet/outlet of the optical waveguide is (the apertures are not necessarily the same), the larger the coupling loss becomes due to leakage of the signal light. In particular, the higher the performance is, the smaller the aperture of the light-receiving portion of the light-receiving element becomes. For example, the aperture of a light-receiving portion of a light-receiving element that achieves optical communication exceeding 20 Gbps is about 30 μm. Thus, the coupling loss becomes large if a common multimode waveguide is used, since the aperture of the outlet is about 50 μm.
The greater the distance between the optical element and the optical waveguide is, the larger the coupling loss becomes since the signal light output from the optical waveguide and that output from the light-emitting portion of the light-emitting element are dispersed. To cope with this, a technology for suppressing the dispersion of the signal light by providing a collecting member of the signal light on the circuit board is disclosed in, for example, Japanese Laid-Open Patent Publication Nos. 2004-241630, 2009-16707, and 2006-47764.
However, in the conventional technology described above, the optical module cannot be easily fabricated due to the complicated fabrication process of the member that suppresses the dispersion of the signal light.